Production of sulphuric acid by the contact process



C. B. CLARK Nov. 5, 1935..

lPRODUCTION OF SULPHURIC ACID BY THE CONTACT PROCSS Filed May 9, 19331NVENTOR. 00P/z 9. 62,47/0

ATTORNEY.

IAII

Patented Nov. 5, 1935 UNITED STATES PRODUCTION OF SULPHURIC ACID BY THECONTACT PROCESS Cyril B. clark, scandale, N. Y., assignor to AmericanCyanamid Company, New York, N. Y., a corporation of Maine ApplicationMay 9, 1933,Serial No. 670,123

V2 Claims.

serious problems in vregard to its safe disposal.'

Attempts have been made to dispose of this sludge by burning it but sucha procedure is generally unsatisfactory due to the sticky, acid, odorouscharacter of the material which makes it diicult `to handle, thecorrosive action of the acid vcompounds on the burner equipmentandthesteam boiler when it is used as a fuel for producing power, and dueto the large volume of gases evolved containing oxides of sulphur andwhich may cause a nuisance in thickly populated communitiesand even inisolated locations may damage vegetation. Due to its obnoxiousproperties it cannot be dumped into streams. The usual practice has,therefore, been to transport it' to some out of the way spot and depositit in some depression far from localities where it can do harm.Therefore the disposal of acid sludge has presented a serious problemto'oi'l reneries.

Several processes have been proposed in the past for utilizingacidsludges. These `processes in general consist in decomposing theacidsludge lby means of heat with concomitant reduction of a greater orlesser proportion of the S04 compounds by the carbonaceous material toform a gas containing sulphur dioxide and the utilization of thissulphur dioxide gas for producing `sulphuric acid. As the demand in oilrefineries is usually for strong acid, the contact sulphuric acidprocess, which produces this grade of acid, is particularly applicablefrom the oil reners standpoint. Until recently, none of the processesproposed have proven commercially successful.

The process described by Hechenbleikner, in'

U. S. Serial No. 568,050, filed October 10, 1931, now Patent1,953,225'is'the rst to achieve notable commercial success. In thisprocess the acid sludge is heated in rotary kilns by direct con-tactwith hot combustion gases which may be produced either from carbonaceousmaterial or if desired, from brimstone or sulphur bearing `materials. Ithas also been recently proposed to ther- .mally decompose certain typesof acid sludge in (Cl. v.Z3-175) externally fired retorts with violentagitation, while `keeping the sludge liquid by diluting with petroleumfractions.

When acid sludge is thermally decomposed `a gas isobtained containingsulphur dioxide, some 5V hydrocarbons or hydrogen containing materialsand in some cases, nitrogen, carbondioxide, etc., together with aconsiderable amount of water vapor. In commercial practice, the watervapor is removed by cooling the gases in a-.condenser and then treatingthem with strong sulphuric acid in a ldrying tower. This operation alsoremoves a large part of the hydrocarbons but unless conducted with thegreatest care and with large and expensive equipment, the removal willnot be complete and some of the more .difcultly condensable hydrocarbonswill remain in the gas. In order4 to supply sufficient oxygen Ytoconvert the sulphur dioxide to Asulphur trioxide, air `is generallymixed with vthe gas before it enters the drying tower and the mixturethen'passes along to the converter in the contact sulphuric acid plant.When the hydrocarbons or other hydrogen containing material in the gasreach the'converter they are oxidized and the hydrogen contained thereinforms water. The present invention relates, particularly to the controlof the amount of water so formed and the modications in the contactsulphuric acid process made necessary by the presence of thesehydrocarbons.

The 'Water formed in the converter does no particularharm to thecatalyst as the temperature is so high that it does not react with thesulphur trioxide formed. However, if the amount of Water exceeds' about30 mg. per cu. ft., serious trouble is 35 encountered after the gasesleave the converter. The gases are :cooled before they pass to anabsorption tower in which the sulphur trioxide is absorbed in strongacid and converted vto sulphuricfacid by combination with water. Ifwater 40 is present and the gases are cooled below the dew point,sulphuric acid mist may be formed and seriously corrode and destroy thecooling equipment and the gas flues. The mist so formed, however, isvery difficult to condense anda large part of it will pass directlythrough the absorber, will corrode the stack and be lost. Even when thegases are caused to enter the absorber at temperatures above the dewpoint, sulphuric acid mist will form when the gases come in contact withthe relatively cold absorbing acid. The sulphuric acid mist thus formedby either step vconstitutes a loss in yield in the process, and,moreover, may cause a serious nuisance in lthe surrounding neighborhoodif discharged into the atmosphere in large amounts.

The prior art discloses no method of handling this acid mist problemraised by the presence of hydrocarbons in the SO2 gas. This is probablydue to the fact that no successful commercial processes for producingSO2 from acid sludge were worked out until the recent Hechenbleiknerprocess referred to above was put into operation. In the Hechenbleiknerprocess, it is suggested that the hydrocarbon compounds be completelyremoved. 'I'his procedure, is, of course, effective in preventingformation of acid mist but it presents certain diiculties in practicaloperation, particularly, when using acidsludge containing a high oilcontent, as it is almost impossible to reliably and continuously removeall hydrogen containing material from the gas stream without suchelaborate equipment as to render the process unnecessarily expensive.

In accordance with the present invention, the problem of acid mist issolved in a simple manner without the use of expensive equipment. I havefound that it is not necessary to completely remove hydrocarbons andsimilar hydrogen containing gases prior to their introduction into theconverters. The free Water in the gases is very efficiently removed by acondenser and drying tower system, to a point where the water content is2 mg. per cu. ft., or less and no trouble is experienced due to theformation of acid mist unless the water content exceeds a certainminimum which I have found in practical operation to be from 25 to 30mg. per cu. ft. If, therefore, hydrocarbons or other hydrogen containinggases are only partially removed, so that the amount of hydrogenremaining does not substantially exceed 21/2 to 3 mg., so that theamount of water formed by burning these hydrocarbons in the converterwill not exceed from 23 to 28 mg. per cu. ft., no difficulty isencountered.

The reduction of the hydrocarbon content of the gases to the desiredpoint can be accomplished in various ways. It may be effected forexample, by a simple form of pre-combustion furnace introduced into thetrain of equipment between the acid sludge still and the converter. Inthis case the air which is ordinarily added to provide the necessaryoxygen for carrying out the contact sulphuric acid reaction can beintroduced into the combustion furnace. Other methods such as therefrigeration of the gases to condense difiicultly condensableconstituents and particularly scrubbing with solvents in which thehydrocarbon gases are soluble are also effective. None of these methods,however, will result in complete removal of all the hydrocarbon gases,as has been suggested in the past by Hechenbleikner, unless practicedwith extraordinary care and with elaborate equipment. It is an advantageof the present invention that contrary to what was thought necessary inthe past, a portion of the hydrocarbon gases may remain in the gasespassing to the converters and not cause any harm, provided their amountsare kept below the critical point. It is, therefore possible to operatewithout elaborate equipment and yet be completely free from the seriousmenace of acid mist formation.

When partial combustion is to be employed in order to lower thehydrocarbon content of the gas stream, the pre-combustion furnace may besituated between the acid sludge still and the water condenser. Thiswill result in the combustion of most of the hydrocarbons bothcondensable and non-condensable. Where the amount of condensablehydrocarbons is comparatively small as in many installations using theHechenbleikner process, on typical low oil sludges, this does not causeany serious problem and the full effect of water removal achieved byusing the condenser and drying tower in series is retained. With somesludges and under certain conditions of operation, however, a largequantity of condensable hydrocarbons are present in the gas stream andin such cases it may be desirable to pass the gas stream first throughthe condenser to remove a considerable portion of the Water and all ofthe condensable hydrocarbons. When the pre-combustion furnace is placedafter the condenser, it throws a somewhat higher water load on thedrying tower. This may make it desirable to insert a second condenserbetween the furnace and the drying tower or if the drying tower acidabsorbs more moisture than is needed to take care of the S03 formed inthe whole system some of the drying tower acid may be sent back into theacid sludge still and redistilled in accordance with the application ofHechenbleikner, Serial No. 574,244, filed November 1l, 1931. If the gasconditions are such that drying tower acid is to be re-circulated in anyevent, the second condenser may be dispensed with, a cooler substitutedand the amount of drying tower acid recirculated slightly increased.

In general, the present invention is not concerned with a particularplacement or arrangement of hydrocarbon removing equipment and includesany suitable equipment or method which will reduce the condensablehydrocarbon content of the gases to the point where they do not exceedthe upper limit herein specified as required to prevent the formation ofacid mist. No claim is made in the present application to the particulararrangement of hydrocarbon removal per se apart from its use in theprocess of the present invention. The invention is also in no senselimited to any particular type of equipment used for the production ofsulphur dioxide containing gases from acid sludge, although it isparticularly applicable and operates effectively with the equipmentdescribed in the Hechenbleikner process.

In its broader aspects, therefore, the invention is not concerned withthe particular design of acid sludge decomposing retorts or kilns norwith the particular design or placement of hydrocarbon removingequipment. In its more specific aspects, however, the combination of thepartial removal of non-condensable hydrocarbons or hydrogen containinggases with the I-Iechenbleikner system. of acid sludge decomposition,constitutes a preferred embodiment and is claimed as a specificmodication of the invention.

Similarly, the invention is not concerned with a particular type ofcombustion gas used for effecting acid sludge decomposition in aHechenbleikner system and is equally applicable to the two mainmodifications of this system, namely, the one using combustion gasesfrom carbonaceous material and the one using combustion gases fromsulphur containing material. Obviously, of course, when sulphurcontaining gases are used, the amount of. surphur dioxide per unitvolume of gases leaving the acid sludge kiln is very much greater,consequently the volume of added air which is necessary before the gaspasses into the converter is similarly increased. This results in a muchlarger dilution of the gases,

of course, that this system which is shown in theY drawing, is a typicalillustration only, and the invention is not limited to the detailstherein set forth.

'Ihe above general discussion of the invention has been based primarilyon continuous processes of acid sludge decomposition, of which theHechenbleikner process is a typical illustration. While the inventioncan be used with particular eiect in conjunction with continuousprocesses where the evolution of hydrocarbon gases is more or lessregular the invention is in no sense limited to continuous processes andis equally applicable to batch processes in which acid sludge isthermally decomposed in batches. Such processes require a somewhatcloser adjustment as the rate at which hydrocarbon gases are given offis not as uniform as in a continuous process.

The following description refers to the attached drawing and disclosesone way in which my invention may be carried out.

Air and fuel enter the combustion furnace I in regulated amounts, thefuel and the necessary air being pumped into the furnace under a slightpressure. The combustion gases from furnace I enter a rotary retort 2which is provided with suitable agitating means. The acid sludge iscaused to travel counter-current to the combustion gases. The sludgeenters the rotary retort at 3 and while passing through the retort issubiected to increasing temperatures in contact with the combustiongases, three main temperature zones being provided, first, a temperaturehigh enough to distill off water and light hydrocarbons, second, atemperature at which most Aof the S04 compounds are reduced to sulphurdioxide and, third, a still higher temperature at which the sludge istransformed to a granular stable fuel. In the drawing the sludge isintroduced continuously through a hollow trunion at 3, and the fuel isdischarged through a discharge valve 4. It should be noted that thetemperature of the different zones 'can be easily regulated and thereactions modified by varying the amount of acid sludge charged, thevolume and/or temperature of the gases entering the rotary retort andspeed of rotation of the retort.

The sulphur dioxide containing gases generated in the retort passthrough a pipe into the combustion furnace 5 and here most of thehydrocarbons are burned. Air and, if necessary, additional fuel aresupplied to the furnace 5, in order to give the necessary oxygen and thenecessary temperature to oxidize the hydrocarbons.

The gases from the combustion furnace pass through the condenser 1,which is cooled by water introduced through the pipe 8, and leavingthrough pipe s. The condensed water is continuously removed through thewater boot or seal Iii. The partially dehydrated gases then pass througha pressure relief valve II into a drying tower I2, where drying acid iscontinuously circulated by means of the acid pump I3. If required,additional air may be introduced through the valved pipe Ill. The gasesnext pass up throughthe drying tower I2 and are dehydrated to the pointwhere they contain approximately 2 mg. of water per cu. ft. They thenpass through the meter I5 to the blower I6.

The dry lgases at the blower are at approximately room temperature dueto the cooling in Y the condenser 'I Vand the drying tower l2. l Thesegases must be heated in order to bring them up to Athe tempera'ture4necessary for the catalytic oxidation of sulphur dioxide to sulphurtrioxide. Therefore, the gases are passed through heat exchanger H wheretheir temperature is raised by heat"y interchange with a portion of thegases which have passed through the converter and which are at a hightemperature due to the heat 1 generated by the -reaction. The hot gasesfrom the oonverterpassthrough the valved pipe I8 into the heat exchangerIl' and thence through the pipe I9 into the line leading from theconverter 2U to the absorption tower 2|. The gases 1 after beingpreheated in the heat exchanger enter the converter through the pipe 22and leave by the T 23.

Strong sulphuric acid is circulated over the absorbing tower 2| by meansof the pump 35 2 through the cooler 24, the excess acid produced beingtaken off through the pipe 25 while weak acid is introduced through thevalved pipe 30 to maintain the strength of the absorbing acid at theproper point. The residual gases substanti- 2 ally free from sulphurtrioxide and sulphuric acid mist pass out of the stack 26 to theatmosphere.

Since the gases leaving the blower I5 are at a low temperature it isimpossible to start up the 3 converter when the plant is to be broughtinto operation without auxiliary heat, because the temperature isinsuflicient to permit the catalytic oxidation of sulphur dioxide tocommence. Therefore, a starting furnace 26 is provided which 3 can beused to send hot combustion gases through the heat exchanger I! and tothe atmosphere through valved pipe 21. As soon as the gases in the heatexchanger have reached the necessary temperature, so that conversionstarts in the con- 4 verter, the valves 2 and 28 are closed and thevalves 29 and 34 are opened and thereafter a portion of the gasesleaving the converter serve to preheat the gases in the heat exchangerI'I.

-The drawing shows the various pieces of equipment in purelydiagrammatical form. Since the present invention is not concerned withdetails of mechanical construction the drawing is really more in thenature of a flow sheet. It should be understood, of course, that variousmodifications in equipment and arrangement may be necessary to meet theparticular conditions in diierent plants, and, of course, are includedin the invention. 5

I claim:

1. A method of producing sulphur trioxide which comprises subjecting anacid sludge from the sulphuric acid treatment of hydro-carbonaceousmaterial to thermal decomposition at 6 a temperature at which S04compounds of the sludge are reduced to SO2, removing the SO2 from thezone of decomposition together with water vapor and gaseous substancescontaining hydrogen, subjecting the said gas stream to de- 6 hydration,incompletely removing substances containing hydrogen capable of reactingwith oxygen to form water to an extent such that the remaining hydrogencontent, after adjustment of the gas stream to the oxygen content nec- 7essary for catalytically oxidizing the SO2 is not greater than 3 mg. ofhydrogen per cu. ft. of gas, heating the gas stream to a temperature atwhich it can be catalytically oxidized and subjecting it at thistemperature to the action of a sulphuric 7 drogen capable of reactingwith oxygen to form water to an extent such that the remaining hydrogencontent, after adjustment of the gas stream to the oxygen contentnecessary for catalytically oxidizing the SO2 is between 0.11 and 3 mg.of hdyrogen per cu. ft. of gas, heating the gas stream to a temperatureat which it can be catalytically oxidized and subjecting it at thistemperature to the action of a sulfuric acid catalyst in a converter,whereby the SO2 is cata- 10 lytically oxidized to S03.

CYRIL B. CLARK.

